Perhaps you should try some spelling variations:
greatest challenge » greatest challenges, great challenge, treatment challenges
challenge funding » challenge finding, challenges finding, challenge facing
4.1 greatest » 4.1 treatment, _ greatest, its greatest
among idp » among idps, among ip, among im
among di » among hiv, among _, among 15
nan yan » nan ya, wang yan, qian yan
yang xi » yang di, yang yi, yang jui
ra 4.1 » era 4.1, rda 4.1, ia 4.1

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: ((nan yan) OR (yang xi))ng ra 4.1 greatest challenge funding ((among di) OR (among idp)) ;~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((nan yan) or (yang xi))ng ra 4.1 greatest challenge funding ((among di) OR (among idp)) ;~".
• Adding a wildcard symbol may retrieve word variants: ((nan yan) or (yang xi))ng ra 4.1 greatest challenge funding ((among di) OR (among idp)) ;*.